Electrotherapy devices for treating tachycardiac arrhythmias are known in the art, for example in the form of implantable cardioverters/defibrillators (ICDs). The devices serve to detect tachycardiac arrhythmias, such as atrial flutter or atrial fibrillation, and to end them, if possible, by means of targeted delivery of a therapy sequence that contains at least one electrostimulation pulse or defibrillation shock. In this respect, the ventricular tachycardiac arrhythmias, such as atrial flutter (ventricular tachycardia) or atrial fibrillation (ventricular fibrillation), are of particular interest. These ventricular tachycardiac arrhythmias can have their origin in the ventricle itself, and are referred to as ventricular tachycardia (VT) in the narrower sense or as ventricular fibrillation (VF). If the ventricular tachycardiac arrhythmia of the ventricle has its origin in the related atrium of the heart or in the sinus node, the ventricular tachycardiac arrhythmias are referred to as supraventricular tachycardia (SVT).
In the case of ventricular fibrillation, uncoordinated contraction of the ventricle in question occurs due to circulatory excitation of the myocardium (heart muscle tissue), leading to the result that the ventricle is no longer able to transport blood. However, some amount of coordinated contraction can still be found in the case of atrial flutter. Atrial fibrillation and atrial flutter are differentiated, for example, in that the frequency of the excitation is still greater in the case of atrial fibrillation than in the case of atrial flutter. Methods of detecting and differentiating the different tachycardiac arrhythmias are known in the art.
Also known are various therapy sequences for terminating a tachycardiac arrhythmia. These include anti-tachycardia stimulation (anti tachycardia pacing or ATP) in the form of an overdrive stimulation, in which stimulation pulses are delivered at a stimulation rate that is elevated as compared with the intrinsic (tachycardiac) heart rate that is present; and delivery of cardioversion shocks or delivery of defibrillation shocks, where the former usually have a lower energy than the latter. Defibrillation shocks are supposed to make the entire myocardium of an affected heart chamber refractory at the same time, and thus temporarily unresponsive to excitation, in order to thereby interrupt a circulating excitation of the heart muscle in question. Other therapy sequences, such as anti-tachycardia stimulation (ATP), include multiple, more energy-rich stimulation pulses for cardioversion of the heart chamber in question. The therapy sequences therefore differ in the number of stimulation pulses or defibrillation shocks delivered, for example, and in the point in time at which individual stimulation pulses or defibrillation shocks are delivered, i.e., at which a stimulation pulse follows a prior stimulation pulse for cardioversion.
If a cardioversion of a ventricular tachycardia (VT/atrial flutter) is supposed to take place in the form of overdrive stimulation as a therapy sequence, stimulation pulses are delivered which have an overdrive stimulation rate that is elevated as compared with the intrinsic (tachycardiac) heart rate. This is intended to interrupt a reentry cycle of excitation of the myocardium, which is typical for ventricular tachycardias (VT/atrial flutter), by means of a stimulation pulse that takes place before the natural (intrinsic) excitation of the heart chamber in question.
From the art, implantable cardioverters/defibrillators (ICDs) are known which are configured to first detect a tachycardiac arrhythmia, and to deliver therapy sequences of different ranking for treating a tachycardiac heart arrhythmia that has been detected. In this respect, first a therapy sequence having a low rank is delivered, which usually includes delivering stimulation pulses having low energy, which are thus more easily tolerated by the patient. If a low-ranking therapy sequence does not lead to successful termination of the tachycardiac heart arrhythmia, a therapy sequence having a higher ranking is triggered. Usually, the therapy sequence having the highest ranking is the (painful) delivery of a defibrillation shock. Furthermore, ICDs are known that are configured to detect ventricular tachycardias and assign them to one of several VT zones. For example, a VT zone may include ventricular tachycardias within a particular heart rate range. The detection of a ventricular tachycardia in a specific VT zone causes these known ICDs to first trigger the therapy sequence that last led to successful termination of the ventricular tachycardia in this VT zone. If this therapy sequence does not lead to therapy success (i.e. does not terminate the ventricular tachycardia), a different therapy sequence is subsequently triggered. If this becomes necessary, the other therapy sequence is used as the first therapy sequence when the next tachycardiac heart arrhythmia in this VT zone is detected.
US 2007/0049974 deals with systems for selecting therapy sequences having different rankings on the basis of a morphology analysis of the electrocardiogram that characterizes the tachycardiac heart arrhythmia.
The known mechanisms for adapting the therapy sequence are therefore restricted to an increase in the shock energy (i.e. the transition from a therapy sequence that is lower in rank to one that is higher in rank) as a reaction to a tachycardiac heart arrhythmia, in the event that the current therapy sequence proves to be unsuccessful; or to one-time delivery of a therapy sequence that previously proved to be successful; or to weakening of a therapy sequence that was repeatedly unsuccessful.